Several types of packaging machines include hollow forming heads that form film into a generally open ended tubular shape so that a first longitudinally extending edge of the film overlies a second longitudinally extending edge thereof. The overlapping edges are then joined to one another by heat sealing or static sealing.
In static sealing, an electrical charge of a first polarity is applied to the first edge of the film, and a charge of opposite polarity is applied to the second edge so that the edges cling to one another as a result of electrostatic attraction.
The conventional means of forming the seal includes a first electrode member positioned outside the generally tubular cavity defined by the film as it exits the hollow forming head. The first electrode member includes an arm member having a return bend formed therein that reaches into the interior of the tubular cavity at the bottom thereof where the opposite longitudinally extending edges of the film are overlapped. A first longitudinally extending edge of the film is captured by the return bend part of the arm member; the arm member overlies the second longitudinally extending edge. A second electrode member is spaced downwardly from the first electrode member and a stream of electrons flows between the first and second electrodes and applies charges of opposite polarity to different ones of the overlapping edges so that said edges cling to one another due to electrostatic attraction.
The primary limitation of the prior art device and method resides within the restrictions imposed by the external positioning of the first electrode vis a vis the tubular cavity defined by the film. Since the first electrode is mounted externally to the tubular cavity, the arm member having a return bend formed therein must be provided. Thus, the transverse extent or depth of the return bend limits the amount of overlap that may be established between the opposite longitudinally extending edges of the film. Since the amount of overlap is limited, the strength of the static seal is limited, i.e., the less the amount of overlap between the oppositely charged edges, the less the strength of the electrostatic seal therebetween.
Moreover, the overlying or first longitudinally extending edge of the film will often be inserted too deeply into the return bend part of the arm member. The film, being very flexible, simply wrinkles and folds over on itself when so inserted. Thus, the strength of the electrostatic seal, already weak due to the limited extent of overlapping imposed by the return bend, is further attenuated by the wrinkled configuration of the overlying edge of the film.
A means and method for applying a static seal in the absence of an externally mounted electrode and its troublesome return bend is needed, but no such means or method appears in the prior art.